"Yep. Mainly started at NASA, but there's good scientific reason to call it that. In brief, we have to get on speed and altitude to be there, so we're on orbit, like being on a perch."

So, the answer for when to use one and the other depends on what you want to say and who your audience is. If you want to give the common meaning of "in orbit", then say "in orbit", but if you are speaking with an audience familiar with the jargon of the space industry, and you want to sound knowledgeable, use "on orbit".

Ooh, I didn't know that at all. Very informative, and it's great to learn something new. Upvoted accordingly! (I am starting to think that the combination of experiments and orbits is a linguistic MINEFIELD, though.)
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thesunneversetsNov 20 '10 at 19:46

3

A ship or satellite is "in orbit" if that's its location. Something done during orbit is done "on orbit". This matches common English parlance. "The train arrived in London". "I'll get some food on arrival."
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David SchwartzJan 31 '12 at 10:52

I think this answer overgeneralizes the meaning of the phrase "on orbit" based on the semantics of the phrase "experiment on". Also, since the phrase "experiments on orbit" sounds wrong/awkward with this parsing unless you change "orbit" to "orbits", it makes it very unlikely that this parsing of the phrase was intended by the author of the question.
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KosmonautNov 20 '10 at 20:25

Yes, I think you have the right of it. I'd never seen the technical usage you describe in your answer.
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resNov 20 '10 at 21:25

It is my understanding that, within the aerospace industry, "on orbit" means "in the correct orbit" (such as "on target" or "on path"). I think "in orbit" is more generic. A spacecraft can be "in orbit", but not in the correct orbit. If it is "on orbit" it is both "in orbit" and in it's intended orbit.

I've thought about this question some more and decided that it's very interesting, so I've upvoted it!

First, I'd like to say that I completely concur with Res's answer. "In orbit" is such a well-used and well-understood phrase in its own right that "experiments in orbit" can only really mean "experiments that take place while circling the earth". Which means that if you want to talk about experiments concerning bodies' orbits, you have to use another word: "about" or "on".

However! Elsewhere, "to experiment in something" does NOT mean to experiment in a location. "Experiments in social marketing" is the same as "social marketing experiments". "Experiments in chemistry" sounds a bit more stilted than "chemistry experiments", but I think most people would agree it means the same thing. So I fully understand why someone might try to say "experiments in orbit", to mean "experiments in the field of orbits". But because it sounds so much like "experiments taking place in outer space", I think native English speakers would steer clear of such a phrasing.

"To experiment on", conversely, conventionally means that you are experimenting in order to see the resulting effects in the object of the experiment. "Experiment on animals" is probably the most common usage of this, sadly. To "experiment on an orbit" might mean to do various things to an orbiting body to see how its orbit changes from one test to the next.

I hope this has clarified things a little bit, and not just confused them further!